Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to an insulated gate bipolar transistor including a freewheeling diode built-in.
Description of the Background Art
Power devices being power semiconductor elements are widely used in the fields ranging from household electrical appliances, electric-powered vehicles, and railroads to solar-electric power generation and wind power generation that increasingly attract attention as “renewable energy.” Inverter circuits are often built in the power devices to drive inductive loads of induction motors in those fields. In this case, freewheeling diodes (hereinafter simply referred to as diodes) for causing currents generated by a counter electromotive force of the inductive loads to flow back are needed, and the inverter circuits are normally formed of a plurality of insulated gate bipolar transistors (hereinafter referred to as IGBTs) and diodes. However, size, weight, and cost reductions of inverter devices are greatly needed, so that a plurality of semiconductor elements are not preferably installed. Thus, as a method for solving this, reverse conducting IGBTs (hereinafter referred to as RC-IGBTs) that include each of the IGBTs and the diodes formed into the same chip have been developed (see Japanese Patent Application Laid-Open No. 2008-53648 and Japanese Patent Application Laid-Open No. 2008-103590, for example). Consequently, reductions in areas on which the semiconductor elements are installed and in costs are promising.
Such RC-IGBT includes the IGBT and the diode formed on one semiconductor substrate, but both of the elements need to be formed simultaneously instead of individually to achieve low costs. An impurity concentration of the outermost surface of an anode diffusion layer directly below an emitter electrode of the IGBT typically needs to be increased. However, recovery properties of the diode are contrarily decreased, so that the impurity concentration of the surface cannot be set sufficiently high.